As the demand for manufactured products increases, so too does the requirement for appropriate tools and methods to produce these products. In a free market society, the competitive system demands that every product be produced at the highest possible quality standards while maintaining the lowest possible cost. To achieve this goal, companies must look to the engineering departments within tool manufacturing companies to develop the proper processes and equipment to bring about the best product or tool design.

What is a Broach Tool Designer?

A broach tool designer is a manufacturing engineer who is concerned with the creation and implementation of the broach tools and associated equipment needed to provide the metal working industry with increased manufacturing productivity while, at the same time, reducing manufacturing costs.

Broach tool design is a specialized area of manufacturing engineering which comprises the analysis, planning, design, manufacture, and use of tools, methods, and procedures necessary for manufacturing productivity. In order to perform these duties, the broach tool designer must have a working knowledge of machine shop practices, tool making methods, machine tool design, manufacturing procedures and practices, as well as the more conventional engineering disciplines of planning, designing, engineering graphics and drawing, and cost analysis.

Responsibilities and Requirements of the Tool Designer?

Broach tool designers are responsible for a wide variety of special tools. Although most of these tools are an end product, some will be designed merely as aids for manufacturing. The broach tool designer must be familiar with the following insofar as they affect the design of broach products and other special cutting tools:

Broach cutting tools

Broach tool pullers and holders

Metalworking fluids and coolants

Jigs and fixtures

Gages and other measuring devices

Nuts, bolts, and various other fasteners

Metallurgy and steel composition

Heat treatment

Machine tool design

Electronics and computers

The current size of the company and the number of tool designers will determine the exact duties of each designer. If many tool designers are employed, each designer may have an area of specialization, such as punch and die design, layout, programming, etc. If there are few tool designers, however, one tool designer may have to cover many areas of tool design.

The Objectives of Tool Design

The main objective of broach tool design is to increase the number of parts produced while maintaining quality and lowering tool costs. To this end, the broach tool designer must realize the following objectives or goals:

Broach tools will be designed for which quality and reliability can be defined and measured in quantitative terms.

Broach tools will be designed with quality parameters that reflect customer needs and usage conditions.

Broach design engineers will provide technical assistance to suppliers and customers for the purpose of promoting tool quality and the quality of the resultant parts.

Broach design engineers will present new products or designs to the market only if the overall quality is superior to that offered by the competition, or if no similar product exists.

Broach tools will be designed that allow customer production rates to increase while still producing acceptable parts.

Broach tools will be designed that require no (or as little as possible) special tooling, thus making the tool more cost effective.

Broach tools will be designed that are safe to operate.

The Design Process

SECTION 1.4The Design Process

While some of the specific aspects of a broach tool will be discussed in subsequent sections, there are a few basic principles and procedures that should be introduced at this point. The design process consists of these basic steps:

Initial summary of the design problem.

Analysis of the design requirements.

Development of preliminary ideas.

Development of alternatives.

Finalization of design.

While these steps are separated for this discussion, in practice each of them actually overlap the others. For example, when stating the problem, the requirements must also be kept in mind to properly define and determine the problem or task to be performed. Likewise, when determining the initial design ideas, the alternative designs are also developed. So, like many other aspects of manufacturing, broach tool design is actually an ongoing process of creative problem solving.

SECTION 1.4.1Summarize the problem

The first step in the design of any broach cutting tool is to define the problem as it exists without tooling. This may simply be an assessment of what the proposed tool is expected to do, such as broach an elongated hole. Or, it may be an actual problem which has been encountered in production where broach tooling may be beneficial, such as low volume production output caused by a bottleneck in a milling operation.

Once the exact extent of the problem has been determined, the problem can be analyzed. Once analyzed, the problem can be resolved throughout the remaining steps of the design process.SECTION 1.4.2Analyze the design requirements

After the problem has been isolated, the specific requirements such as function, quality, cost, delivery, and other related specifics can be used to determine the specific parameters within which the designer must work. Every broach tool that is designed must (1) perform certain functions, (2) meet certain precision and accuracy requirements, (3) keep the cost to a minimum, (4) be available when the customers’ production schedule requires it, (5) operate safely, and (6) meet various other requirements, such as adaptability to the machine on which it is to be used, or have an acceptable working life.

Figure 1-1 shows a method of applying these criteria to the process of choosing a tool design. Rarely, if ever, will one tool design be best in each of the five areas shown. The broach tool designer’s task here is to weigh all these factors and select the design that best meets these criteria and the task to be performed. As you can see in the figure, option A presents the best alternative (the highest total provides the best option; the higher the value of each factor, the more appealing that factor is to the customer). Although option B rates highly in terms of function and quality, the cost and delivery make it prohibitive. The opposite is true for option C.SECTION 1.4.3Develop preliminary ideas

Initial design ideas are normally conceived after an examination of the preliminary data. This data can be seen on the Key Information (New Broach Designs) form at the end of this section. While evaluating this information, the broach tool designer should take notes to insure that nothing is forgotten during the initial evaluation. Should the tool designer need more information than that furnished with the design package, the sales agent, engineer, or planner responsible for the tooling request should be consulted to determine the required additional information. In many cases, the tool designer and planner must jointly develop parameters during the initial design phase.SECTION 1.4.4Develop design alternatives

During the initial concept phase of design, many ideas will occur to the tool designer. As these ideas are thought out, they should also be written down so they are not lost or forgotten. There are always several ways to do any job. As each method is developed and analyzed, the information should be added to the list shown in Figure 1-1.

Figure 1-1.
A basic model for tool design analysis

SECTION 1.4.5Complete design ideas

Once the initial design ideas and alternatives are determined, the broach tool designer must analyze each element of the design to determine the best possible way to proceed toward the final tool design. As was stated earlier, rarely is one tool alternative a clear favorite over any other. Rather, the designer must evaluate the strong points of each alternative and weigh this value against the weak points of the design. For instance, one design may have a high tool life but the cost of the tool may be very expensive. On the other hand, a second tool may provide less life but will cost much less to build. In this case, the value of the production rate might be the factor used to determine which will be the best design for the job.

If the job is a long term production run, the first broach tool may pay for itself in increased tool life. If, however, the production run is short or is a one time run, the second broach tool may work best by sacrificing tool life for a reduced tool cost. Seldom will one tool be able to meet all the expectations of the tool designer. In most cases the best design is a compromise of the basic criteria of function, quality, cost, due date, safety, and other requirements.

Tool Drawings

SECTION 1.5 Tool Drawings

A broach tool designer must have a strong background in drafting and mechanical drawing to properly present design concepts to the people who will be making the proposed tool. For the most part, broach makers do not require the same type of drawings as do the less experienced machine operators in a production environment. For this reason, tool drawings can be drawn much simpler and quicker to keep the design cost to a minimum. The following are a few basic points to remember when creating or reviewing tool drawings.

Draw and dimension with due consideration for someone using the drawing to manufacture a tool.

Do not crowd views or dimensions.

Analyze each operation to be sure it can be done with available (or easily obtainable) equipment.

Do not put process information on the drawing. If a specific operation is required, create a process sheet to accompany the design print.

Use only as many views as necessary to show all required detail. Excess section views complicate a drawing.

Tolerances and fits peculiar to broach cutting tools require extra thought. It is not economical to “over-tolerance” a feature that does not require it.

The broach tool drawing must indicate all sizes required to obtain stock for the tool. It is necessary to allow material for finishing in almost all cases. As far as possible, stock sizes known to be on hand should be used.

Use notes only to convey ideas that cannot be communicated by geometric dimensioning and tolerancing (ANSI Y14.5M-1982) and ordinary drawing.

Secondary operations, such as machining of edges, polishing, or similar specifications should be kept to a minimum. Only employ these operations when they are important to the overall fit or function of the tool; otherwise these operations will only add cost, not quality to the tool.

Apply tolerances realistically (see Section 1.6). Overly tight tolerances can add a great deal of additional cost with little or no added value to the tool. The fit or function of the tool feature should determine the specific tolerance, not a standard title block tolerance value.

If tooling is to be duplicated in whole or part or if multiple tools are required, drawings should be well detailed.

SECTION 1.5.1Engineering drawing review

Engineering drawings which are supplied by the customer will include significant characteristics shown on the control plan. However, when no customer engineering drawing exists, as in the case of proprietary designs, the controlling drawings must have written approval by the customer in order to verify which dimensions affect fit, function, and durability, as well as governmental regulatory or safety requirements. These features can usually be found in a customer engineering specification, through previous service or warranty experience, or on assembly drawings.

In any case, all drawings should be submitted to the design review process to determine if there are sufficient dimensions to manufacture the tools without the operator having to calculate any dimensions. (This will greatly facilitate set-up, reduce operator error, and minimize inspection error.) All dimension, control, and datum lines should be clearly identified so that the appropriate functional micrometers, gages, and blocks can be made available for ongoing controls. All dimensions should be evaluated to determine if they are containable and compatible with available manufacturing capabilities.SECTION 1.5.2Drawing and change control

The broach tool design engineering department is responsible for providing the customer with current engineering drawings and specifications as outlined in the purchase order. The designer is responsible for understanding the customers’ part requirements. Joint planning involving the designer, sales representative, and the customer may be required in order to establish a clear comprehension and interpretation of blueprints, part prints, specifications, materials, manufacturing methods, checking methods, and significant characteristics. If any questionable areas exist, the designer must contact the customer immediately for clarification. Written approvals of a new design (especially approval of the characteristics which are significant to the customer) are to be received from the customer before the design can be released for manufacturing.

The broach tool design engineering department must maintain the latest engineering drawings and specifications and insure that all of the necessary engineering documents are available in the event of customer inquiry, customer changes/revisions, or design obsolescence. Please note the following:

Manufacturing notes added to blueprints or process sheets during the manufacturing process must be initialized by the operator making the addition. All manufacturing notes added must be made permanently on the manufacturing process sheets during the print review stage of a reorder.

Process or drawing changes/revisions made during the manufacturing process must be initialized by a design engineer. All additions and changes must be made permanent on the engineering drawing or process sheet during the print review stage of a re-order.

Any obsolete information must be removed from all points of use and either destroyed or identified to prevent misuse of obsolete information. All design changes affecting customer part dimensions or set-up must have written customer approval prior to being incorporated into manufacturing.

Selection of Tolerances

SECTION 1.6Selection of Tolerances

The selection of tolerances has a dual effect on both economics and on quality. The tolerance effects:

SECTION 1.6.1Methods of selecting tolerances
In theory, the designer should, by scientific study, establish the proper balance between the value of precision and the cost of precision. In practice, the designer is unable to do this for each tolerance–there are too many quality characteristics. As a result, only a minority of tolerances are set scientifically. Some scientific tools for tolerancing include:

Regression studies. For example, a tool may be required to produce a feature at a certain dimension with a specified micro finish. Several tools are built and used. The test data recorded are (1) the dimension produced, (2) the micro finish produced, and (3) the physical characteristics of the tool which affect the test criteria. These data permit scatter diagrams to be prepared and regression equations to be computed to aid in establishing tolerances on a basis which is scientific, within the confidence limits for the numbers involved.

Tolerances for interacting dimensions. Numerous designs involve “interacting dimensions”. A cutting tool may consist of a chain of 10 dimensions, located about a single centerline. A pot assembly may consist of a buildup of 15 elements. What they have in common is the existence of interaction among these elements or dimensions. Each element or dimension has its own tolerance. However, the variation of the composite (the tool or the assembly) will be related to the variations of the elements according to the laws of probability; i.e., it is very unlikely that all the extremes will come together simultaneously. This unlikelihood makes it possible to establish wider tolerances on elements of such designs without significantly increasing the extent of non-conformance.

Most tolerances are established by methods which, in varying degrees, are less than scientific. The principal methods include: precedent, bargaining, and standard tolerance levels which have been defined at the company, industry, national, and international levels.

SECTION 1.6.2Unrealistic tolerances

In some companies, the accumulated specifications contain an extensive array of unduly tight tolerances, i.e., tolerances not really needed to achieve fitness for use. Whenever this condition creates problems in manufacturing, the toolmakers will often respond by exceeding the tolerances to meet delivery dates. This becomes a situation of “unrealistic tolerances loosely enforced”. Such a situation has evolved from many historic forces and unfortunately has resulted in distrust between design and manufacturing departments. More recently, companies have tried to reduce the distrust in order to convert to a situation of “realistic tolerances rigidly enforced”.

A conservative approach in the short term is to make provisions for handling specifications having unrealistic tolerances. In the long term, the evolutionary solution to unrealistic tolerances lies in providing designers with the information needed to set realistic tolerances. This includes/ process capability data and data on the cost of achieving various leve

Design Review

SECTION 1.7 Design Review

Design review is a technique for evaluating a proposed design to assure that the design (1) will perform successfully during use, (2) can be manufactured at low cost, and (3) is suitable for prompt, low-cost maintenance.

The concept of design review is not a new one. However, in the past the term has referred to an informal evaluation of the design. New products often require a more formal program. A formal design review recognizes that many individuals involved in the design phase do not have specialized knowledge in reliability, maintainability, safety, producibility, and the other parameters that are important in achieving an optimum design. The design review aims to provide such knowledge.SECTION 1.7.1Design review tools

The design review process can be aided by using some of the following tools:

Seriousness classification of characteristics. Such classification is typically divided into three or four categories, e.g., critical, significant, secondary, incidental. Availability of such a classification simplifies the job of identifying the vital few tolerances. (For elaboration, see Section 1.8.)

Studies of process capability. The accumulated process capability studies can be organized into tables that show, for the principal processes, their inherent uniformity. These tables greatly simplify the job of predicting whether those processes can hold the proposed tolerances.

Charts of cost precision. Data showing the costs of performing operations at various levels of precision is beneficial. It is possible to organize the data in ways which show the correlation between precision and cost. Such relationships are of obvious value.

SECTION 1.7.2The concept of design review

The design review is based on the following concepts:

Design reviews are mandatory, because of upper-management policy declaration, and by customer demand.

The design review is conducted by a team consisting mainly of specialists who are not necessarily associated with the development of the design. These specialists must be experienced and bring with them the reputation of being objective. The combination of competence, experience, and objectivity is essential to prevent the design review program from deteriorating.

As much as possible, design reviews are made to defined criteria. Such criteria include customer requirements, internal goals, experience with previous products, etc.

SECTION 1.7.4Engineering specifications

A detailed design review and understanding of the controlling specifications as supplied by the customer will help the design engineer identify the functional and durability requirements of the tool or assembly which the customer requires. In some cases, acceptance criteria are called out on the engineering specification (usually a blue print of an existing design) provided by the customer. In other cases, all that is provided is a part print, and design engineering must rely on prior experience, Design FMEAs, or Design of Experiments to arrive at and determine which characteristics will affect or control the results that assure meeting function and durability requirements after the product is designed.SECTION 1.7.5Material specifications

In addition to drawings and performance specifications, material specifications should be reviewed for significant characteristics relating to properties, performance, handling, and storage requirements. These characteristics should also be included in the control plan.SECTION 1.7.6Tolerance review

In tolerance review, as in the complete design review, there are human problems as well as technological problems. A major human problem is the impact of the concept of design review on the traditional “monopoly” of the designers. Historically, they consulted manufacturing people only as they felt the need–the choice was still the designers. Under the new concept of mandatory design review the choice is made for the designers–there will be design review and you will participate. The benefits will only be realized when this “cultural resistance” has been broken.

Identification of Significant Characteristics

SECTION 1.8 Identification of Significant Characteristics

Significant characteristics can be defined as the product, process, and gaging requirements which are necessary to insure customer satisfaction (and which the quality manager must have summarized on the Control Plan). The engineering department has the responsibility to identify significant characteristics as determined by the customer for the tools which the customer will use.

Most tool characteristics are non-functional. However, some of them carry tolerances which may involve high costs. The design review is one means of looking for these, through comparing the tolerances with the known process capabilities. The identification may also come from past experience with similar designs and tolerances, i.e., such tolerances have generated high internal rejection rates, high inspection costs, high supplier costs, etc. It is helpful if the design review can identify the potential recurrence of such difficulties. In some cases the designer will be able to provide relief. In other cases it may be possible to modify the process in ways which make it easier to meet the tolerances economically.SECTION 1.8.1 Defining significance

Quality characteristics are decidedly unequal in their effect on fitness for use. A relative few are “significant”, i.e., of strict importance. Clearly, the more important the characteristics, the greater should be the attention they receive in such matters as extent of quality planning; precision of processes, tooling, and checking equipment; strictness of criteria for conformance; etc. However, to make such a discrimination requires that the relative importance of the characteristics be made known to the various decision makers involved: the customer, engineers, quality, inspection, etc. To this end, the cutting tool manufacturer should utilize a formal system of seriousness classification. The resulting classification finds use not only in inspection and quality planning, but also in specification writing, vendor relations, product audits, quality reports, etc. These uses of seriousness classification dictate that the system must:

Decide how many classes or strata of seriousness to create (usually three or four).

Define each class.

Classify each characteristic into proper class of seriousness.

SECTION 1.8.2 Classifying characteristics

The system of classification was pioneered in the Bell System during the 1920s. Most subsequent sets of definitions and classifications have evolved from this venture. Classifying characteristics yields some welcome results by discovering misconceptions and confusion among departments, thereby opening the way to clear up vagueness and misunderstandings.

A problem often encountered in the practice is the reluctance of the designers to become involved in seriousness classification of characteristics. The designers may offer plausible reasons, such as: all characteristics are significant or, the tightness of the tolerance is an index of significance, etc. Yet, the real reasons may be the unawareness of the benefits; other matters have higher departmental priority, etc. In such cases it may be worthwhile to demonstrate the benefits of classification by working out small-scale examples, such as the one shown.

Class

Nature

Description

Examples

Significant

Essential to fit or function

Defects which can render the part unacceptable for use, or prevent the tool from being used.

Dimension over pins

Puller or retriever dimensions

Apex dimensions

Runout or concentricity

Secondary

Appearance

Non-significant defects which might be a detraction to the user, or might have long-term effects on the longevity of the tool.

Gouge marks

Teeth burned by sharpening or grinding

Incidental

Accuracy is not essential

Defects which do not affect fit or function, and are not essential to the user.

Lead chamfer widths

Type of amount of relief

Tooth spacing (unless resharpened on a CNC grinder)

Undercuts or clearances

Figure 1-3.
Seriousness classification of broach defects

Designing for Product Reliability

SECTION 1.9 Designing for Product Reliability

In June, 1957, the historic “AGREE” report, published by the Secretary of Defense revealed some remarkable facts. During the previous decade, costly failure problems were experienced on military electronic equipment and space products. Initially, it was suspected that most of the failures were due to manufacturing or inspection errors. The report revealed that design was the major problem. Fitness-for-use problems in the field showed this breakdown: 40% due to design, 30% due to manufacturing, and 30% due to field conditions (faulty maintenance and improper operation of the product).

As the problem was analyzed, the term reliability emerged. In the “AGREE” report, reliability was defined as “the probability of a product performing without failure a specified function under given conditions for a specified period of time”. More simply, reliability is the chance that the product will work. If this definition is dissected, four implications become apparent:

The quantification of reliability in terms of a probability.

A statement defining the successful performance of our product.

A statement defining the environment in which our product must operate.

A statement of the required operating time between failures.

SECTION 1.9.1The product reliability program

In order to achieve high reliability, it is necessary to develop a reliability program, or define the specific tasks required. The reliability program should emphasize more than just the design phase; manufacturing and field-usage phases should be included so that the program spans the full product life cycle.SECTION 1.9.2Quantifying reliability

The significant aspect of reliability is its quantification. The act of quantification makes reliability a design parameter just like determining pull force or tensile strength requirements. Quantification also helps to refine certain traditional design tasks such as selecting land widths or the amount of back taper to use.

As experience is gained in quantifying reliability, we will learn that it is best to create an index that uniquely meets the needs of those of us who will use the index. Users of the index not only include internal technical personnel, but also marketing personnel and users of the product. Examples of reliability indices and goals are shown in the Figure 1-4.

Figure of Merit

Meaning

Mean Time Between Repairs (MTBR)

Mean time between repairs of a tool that is still in a repairable condition

Repair Rate

Number of total repairs in the tool’s life

Mean Time to Failure (MTTF)

Mean time to failure of a tool not intended for repair

Mean Life

Mean value of life (life may mean wear-out or major reconditioning)

Mean Time to First Repair (MTFR)

Mean time to the first repair of a repairable tool

Longevity

Wear-out time for a tool

Availability

Operating time expressed as a percentage of operating and repair time

System Effectiveness

Extent to which the tool achieves the requirements of the user

Probability of Success

Same as reliability (but used for “one-shot” or non-time-oriented tools)

B50 Life

Life during which 50% of the tool has been used, or median life

Repairs / 100

Number of repairs required per 100 operating hours

Figure 1-4.Examples of reliability indices

Note that all of these examples quantify reliability. Setting realistic reliability goals requires a meeting of the minds on (1) reliability as a number, (2) the environmental conditions to which the numbers apply, and (3) a definition of successful product performance. This is a difficult accomplishment! However, the act of requiring designers and users to define with precision both environmental conditions and successful product performance forces the designer to understand the design in much greater depth and the user to understand the significance of the circumstances in which the product is used.

Design Failure Mode and Effect Analysis

SECTION 1.10 Design Failure Mode and Effect Analysis

Failure mode and effect analysis (FMEA) provides a methodical way to examine a design for possible ways in which failures can occur. In the FMEA, a product is examined for all the ways in which a failure may occur. For each potential failure, an estimate is made of its effect on the total system and of its seriousness. In addition, a review is made of the action being taken (or planned) to minimize the probability of failure or to minimize the effect of failure.SECTION 1.10.1An analysis of the design FMEA

The analysis is elaborated to include such matters as:

Safety. Injury is the most serious of all failure effects. In consequence, safety is handled through special programs.

Effect on downtime. Must the system stop until repairs are made, or can repairs be made during an off-duty time?

Access. What hardware items must be removed in order to get at the failed component?

Repair planning. This includes/ repair time, special repair tools, etc.

Recommendations. Suggestions for changes in designs or specifications, for added tests, for instructions to be included in manuals of inspection, operation, or maintenance.

SECTION 1.10.2How to create a design FMEA

Due to continually changing customer needs and expectations, the importance of a disciplined technique to identify and prevent potential problems is quite great. In the design FMEA, a ranking procedure has been applied in order to assign priorities to the failure modes for further study. The ranking is twofold: (1) the probability of occurrence of the failure mode, and (2) the severity of the effect. For each of these, a scale of 1 to 10 is used. If desired, a risk-priority number can be calculated as the product of the ratings. Priority is then assigned to investigating failure modes with high risk-priority numbers.

In its most rigorous form, the design FMEA is a summary of the design engineer’s thoughts, including an analysis of every item that could go wrong based on experience and past problems. This systematic approach parallels and formalizes the mental disciplines that the design engineer normally goes through in the design process for the product (reference the Design FMEA Procedure flow diagram in the appendix section).

Qualifying Suppliers

SECTION 1.11 Qualifying Suppliers

In our company, the cost of purchased materials, parts, and services sometimes exceeds 50% of the manufacturing costs. Thus, the overall program for quality in our company must extend to the suppliers (or vendors) from whom the purchases are made.SECTION 1.11.1Vendor Quality Policy

Multiple sources of supply will be developed for all important purchases or services.

Suppliers will be required to review all requirements and agree to them before a purchase order is issued.

Where appropriate, the facilities and personnel of our company will be made available to suppliers to assist them in achieving mutual objectives.

Data on supplier quality will be collected and used to determine the parameters in which incoming inspection will operate.

Supplier quality ratings will be prepared for use in selecting suppliers.

SECTION 1.11.2Vendor Quality Policy-Technological

For traditional and proprietary products, the supplier is usually self-sufficient. The products which require outside manufacturing may require the type of engineering and technical assistance given to an in-house department. This assistance may require “exchange visiting”, i.e., mutual visits to see each other’s operations. These visits create the risk that the visitors will make unauthorized use of the knowledge obtained during the visit, but usually the need to take the risks is justifiable.SECTION 1.11.3Vendor Quality Policy-Economic

The life-cycle-cost concept requires that the supplier understand the buyer’s cost over the entire useful life of the product. To the purchase price the buyer must add a whole array of quality-related costs: incoming inspection, material review, production delays, downtime, extra inventories, etc. However, the supplier also has a set of costs which he or she is trying to optimize. The buyer should put together the data needed to understand the life-cycle-costs or the cost of use and then press for a result that will optimize these.SECTION 1.11.4Vendor Quality Policy-Managerial

Because the products or services which we purchase involve a wide range of supplier capabilities, the planning for use of these capabilities must be coordinated with the capabilities of the buyer. A major effect of this form of interdependence is that the assurance of good quality can not be derived from incoming inspection. Instead, the assurance must come from placing responsibility on the supplier to (1) provide the correct product or service, and (2) furnish proof that it is right.SECTION 1.11.5Vendor Quality Responsibilities

The question has arisen: Who is responsible for vendor quality? Shown on the nest page is a chart that outlines separate responsibilities by department (reference the Quality Manual)

Customer Relations

SECTION 1.12 Customer Relations

Ultimately it is the customers who dictate the success of our company; if no one needs the product or service we provide, we will go out of existence. Customers create the demand for our goods and services. America is still a market economy… the customer is still “king”. The two oldest mistakes in customer relations are (1) taking our eyes off the horizon and focusing on the next quarter’s profits, and (2) combining this fiscal myopia with a virtual disregard for our customers. A dedicated effort must be made to strengthen all aspects of the customer-supplier relationship.

SECTION 1.12.1 Applications engineering…the technical relationship

Applications engineering is a form of technical assistance provided to the user by us, the manufacturer. The name is derived from the emphasis placed on helping users select the product design best suited for their needs.

ACTIVITY

Design

Purchasing

Quality Control

Establish a supplier quality policy

*

*

**

Use multiple suppliers for major procurements

**

Evaluate quality capability of potential suppliers

*

**

Specify requirements of suppliers

**

*

Conduct joint quality planning

*

**

Conduct supplier surveillance

*

**

Evaluate delivered product

*

**

Conduct supplier improvement programs

*

*

**

Use supplier quality ratings in selecting suppliers

**

*

** = principal responsibility

* = collateral responsibility

Figure 1-6. Responsibility matrix for determining suppliers

Selecting the most applicable product design requires that the engineer understand a good deal about the problems of the user, including economics. In this way the engineering concept is one of the means available for optimizing user costs. In practice, this form of technical assistance does not stop with the sale of the product; it remains available to help the users if they encounter trouble during use. In providing this assistance during use, the engineer is well placed to provide an added useful feedback to our company. Since it is our policy to provide both the product and the service (engineering assistance), our prices will reflect this.

SECTION 1.12.2 The marketing of quality…the commercial relationship

The “commercial” relationship between our company and our customers is carried out by the various arms of the sales department. They submit bids, negotiate contract terms, fill orders, provide customer service, etc. Product quality plays a role in all of these activities.

Marketing strategy is greatly aided by quality superiority. Marketers are well aware of this and are always urging designers and producers to come up with quality superiorities which can secure a better share of the market or higher prices. Marketers also devote much effort to propagandize those qualities for which their product is superior, in their personal selling.

All other things being equal, superior quality can be converted into higher share of market or higher prices, provided that the customer accepts the presence and usefulness of this quality superiority. However, no manufacturer is able to attain quality superiority for all product features. Hence, each competitor propagandizes those features for which its product is superior. The simple rules relating quality to share of market are as follows:

When a product clearly has superior quality which a customer accepts (or can be induced to accept) as present and useful, then market share is decided primarily by that quality superiority, all other things being equal. It is common for industrial customers to seek out the quality differences in products supplied by competing vendors. Sometimes these differences are quantified through systems of quality ratings. The resulting ratings are then used as inputs to the decisions of how to allocate the available business among the competing vendors.

When there is to quality superiority which the customer accepts as present and useful, share of market is decided primarily by superiority in the marketing skills of the competing marketers. Significant quality differences are often translated into price differences. A common example is the price differential system widely used for product grade differences (such as steel). In addition, there is the wide use of price differentials to charge for added special features or options of the product.

Before we can proceed with a detailed discussion on the functions of a cutting tool, we should take a look at how a cutting tool cuts. As in any metal-cutting operation, broach cutting tools separate small segments or chips from the workpiece (or part) to produce the required shape and size of the manufactured part.

Chip Formation

SECTION 2.1Chip Formation

Chip formation involves three basic requirements: (1) the cutting tool must be harder than the part material, (2) there must be interference between the tool and the part as designated by the feed rate and cut per tooth, and (3) there must be a relative motion or cutting velocity between the tool and workpiece with sufficient force to overcome the resistance of the part material.

As long as these three conditions exist, the portion of the material being machined that interferes with the free passage of the tool will be displaced to create a chip. Many combinations exist that may fulfill such requirements. Variations in tool material and tool geometry, feed and depth of cut, cutting velocity, and part material have an effect not only upon the formation of the chip, but also upon cutting force, cutting horsepower, cutting temperatures, tool wear and tool life, dimensional stability, and the quality of the newly created surface.

The Mechanics of Chip Formation

SECTION 2.2The Mechanics of Chip Formation

Empirical metal-cutting studies reveal several important characteristics of the chips formed during the broaching process: (1) the cutting process generates heat, (2) the thickness of the chip is usually greater than the thickness of the layer from which it came, (3) the hardness of the chip is usually much greater than the hardness of the parent material, and (4) the other three relative values are all affected by changes in cutting conditions and in properties of the material to be machined (see Figure 2-1). These observations also indicate that the process of chip formation is one of deformation or plastic flow of the material, with the degree of deformation dictating the type of chip that will be produced

Plastic Deformation

SECTION 2.3Plastic Deformation

Originally, it was thought that chips formed in metal cutting were created in much the same way that wood chips are formed when split by an axe. This may be partially true for brittle materials such as cast iron, but it does not hold true for the majority of metals. The process by which chips are formed with metal-cutting tools is called plastic deformation, and was first described by Rosenhain at the Stratsfordshire Iron and Steel Institute in 1906.

What actually happens in this shearing process is that the metal immediately ahead of the cutting edge of the tool is severely compressed resulting in temperatures high enough to allow plastic flow. When the resisting stresses in a material exceed their elastic limit, a permanent relative motion occurs and further deformation is withstood. This strengthening is called work or strain hardening, and is characteristic of all steels, but demonstrated most dramatically in stainless steels

How and Where Heat is Generated

SECTION 2.4How and Where Heat is Generated

The force or energy that is put into the tool creates movement in a group of metal atoms in the workpiece. This group is a finite number of atoms which are forced to change their positions in relationship to each other. As the atoms in the metal ahead of the tool are disturbed, the friction involved in their sliding over one another is thought to be responsible for 60% or more of the total heat generated. This internal friction, and the heat it generates, can be compared to the friction and heat caused by bending a paper clip back and forth until it breaks.

As the tool continues to push through the work piece, a chip eventually slides up the cutting face of the tool. This sliding creates an external friction which again releases heat. This external friction accounts for about 30% of the total heat generated.

The third area of heat generation is on the land or flank of the tool. This area accounts for about 10% of the heat generated. This is assuming that the tools are sharp and made correctly as far as clearance angles and face angles are concerned. As the tool wears, the above percentages will vary, especially when there is excess wear on the land, or if the clearance angle is insufficient for the material or the part configuration. This contact zone will actually increase as the part continues to close in after the cut resulting in extremely high pressures on the land area of the tool.

Figure 2-1.
The effects of friction upon amount of chip distortion.

The Built-Up Edge

SECTION 2.5The Built-Up Edge

Another phenomenon we must consider is the built-up edge. As the cutting tool continues to cut into the work piece, the tool face and flank are chemically cleaned as is the metal which is being rubbed against these surfaces. Investigations have shown that freshly machined surfaces brought into rubbing contact with each other, in the absence of any contaminant, undergo metal to metal seizure at the point of actual contact.

In most cases, it is virtually impossible to prevent some amount of seizure between the chip and the tool face. Unless surfaces are perfectly flat, contact is made along the high spots over only a fraction of the total area. As the chip passes over the tool face, cutting forces give rise to extremely high unit pressures, sufficient to form pressure welds. If these welds are stronger than the ultimate shear strength of the material, that portion of the chip which is welded to the tool shears off as the chip is displaced and becomes what is called a built-up edge.

The built-up edge is a transient wedge-shaped mass of metal which is usually harder than the work piece itself. Actual hardness varies with the work-hardening characteristics of the metal being machined. A built-up edge can be helpful or detrimental to the tool. If the built-up edge is large and flat along the tool face, it decreases the effective face angle which makes it necessary to exert more power to cut the metal and, therefore, causes more heat to be generated. If, on the other hand, the built-up edge is small and sharp, it increases the effective face angle and lowers the required force, generating less heat. In addition, the built-up edge helps protect the cutting edge of the tool. The tool merely supplies the foundation for the false cutting edge which is actually doing the work.

It is interesting to note that the built-up edge provides the reason why some coolants work better than others. The correct coolant introduces the correct contaminant to prevent the clean metal to metal contact. One of the functions of the cutting fluid is to optimize the amount of this built-up edge for a given face angle.

The Mechanics of the Cutting Action

SECTION 2.6The Mechanics of the Cutting Action

An analysis of the mechanics of cutting reveals a couple of interesting things about the shearing process responsible for chip formation.

For any given metal, cutting tool, and metal-removal rate (or depth of cut), the amount of heat produced in the shear zone depends essentially upon the size of the shear angle. If we think of the plastic flow, or shear, as taking place along a single plane, the shear angle is the angle between that plane and the direction of tool travel. If this shear angle is small, the plane of deformation runs a considerable distance ahead of the tool, causing the layer of metal removed to be deformed into a short, thick chip. This is severe cutting action, requiring lots of power, and generating considerable heat due simply to the total number of metal atoms being slid over each other. In addition, the built-up edge will be large and floppy resulting in a poor surface finish.

If the shear angle is large, however, the shear path will be short. This causes the built-up edge to be small and controlled. It has been proven that the size of the shear angle is controlled directly by the coefficient of friction between the sliding chip and the tool face. The lower the friction, the less drag the tool exerts on the chip, and the larger the shear angle. In other words, if the friction between the chip and the tool face is reduced by the introduction of a lubricating type cutting fluid, then the heat coming from the rubbing friction will be reduced. More importantly, the reduction in friction causes the shear angle to become larger, allowing the chips to slide easier and consequently reduce the heat coming from the plastic-type deformation, the major source of heat. This is, of course, assuming that machining speed is held constant.

The Effects of Manipulating

SECTION 2.7The Effects of Manipulating

Certain manipulating factors provide some control of the metal cutting characteristics. Some effects of these factors will be discussed herein. The results described have been derived from machining various carbon and stainless steels with sharp high-speed steel broach cutting tools.SECTION 2.7.1Velocity (machining speed)

Velocity affects temperature, which in turn affects the cutting process. At low velocities, the temperature at the tool point is below the recrystallization temperature of the material. As a result, work hardening in the chip is retained and the workpiece material is not softened due to failure to reach the yield strength temperature of the material. If the velocity increases to the point where the cutting temperature is above the yield strength temperature of the material, the chip at the interface tends to soften and machine much more efficiently. Higher shear angles occur at higher velocities and an ideal chip thickness of 1.5 times the depth of cut can be approached. Excessive velocity will cause the tool to fail rapidly since speed has the greatest effect on tool life.

Chip form and shape at high velocities can be very troublesome on ductile materials. The reduced resistance to chip flow and the resultant increase in shear angle gives a thinner, less distorted chip, but one which becomes longer and straighter as velocity increases. Tooth gullets and geometries can be employed to deal with this problem, however.SECTION 2.7.2Depth of cut

Changes in the depth of cut effectively change the cross-sectional area of chip contact. How this area is changed determines the effect upon the cutting process. An increase in depth of cut widens the area of contact on the tooth face and changes the force. This results in greater chip distortion and reduced tool life, although increased depths reduce the machining cycle.

Several factors affect surface quality to a greater degree than may be predicted. Lack of rigidity will permit greater deflections as a result of higher forces. Increases in depth of cut may then cause chatter, poor surface quality, and loss of dimensional stability. Deep cuts on relatively small diameters might result in erratic tool life behavior and poor surface quality. The effect upon the chip is more pronounced with increases in the chip width than with increases in the chip depth. Because of the greater distortion caused by wider chips, width control (by the use of “chip breakers”) is essential.SECTION 2.7.3Tooth geometry

For given cutting conditions, changes in tooth geometry have two direct effects on chip formation: (1) effect upon shear angle, and (2) effect upon chip thickness. The two are related in that a change in one usually affects the other.

The effects of changes in face (hook) angles are the most apparent. The lower hook angles decrease the shear angle, cause greater chip distortion, and increase the resistance to chip flow. Lower (and negative) hook angles produce rougher and more work-hardened surfaces. At low or negative hook angles, the chip is so highly distorted that it is sometimes broken into short lengths.

Figure 2-2.
A view of standard broach tooth geometry.

SECTION 2.7.4Tool material

One effect of tool material lies in its ability to sustain high cutting velocities, as for example between high-speed steel and carbides. The effect of high velocity has already been described. Another factor is the coefficient of friction between chip and tool material. Usually, this is of little consequence with high-speed tools because the coefficient of friction does not change appreciably among the various grades.SECTION 2.7.5Cutting fluids

Ideally, if a cutting fluid provides lubrication between the chip and the tool, the coefficient of friction will be reduced and the shear angle will be increased. However, effective lubrication may be difficult to achieve except possibly at very low cutting speeds. Lubricant effects vary with cutting conditions and with work materials.

At high-speed, fluids act principally as coolants, but may effectively lubricate the tool-chip interfacial zone providing more efficient machining which often results in increased tool life and improved surface finish. Constant, even flow is essential when cutting fluids are applied with carbide tools to prevent thermal shock and resultant fractures. (For a fuller treatment of the subject of cutting fluids, see Section 3.0.)SECTION 2.7.6Workpiece materials

Brittle materials form discontinuous chips, and can be broached with decreased hook angle and no chip breakers. Cutting forces are usually lower for cast iron (and other high carbon steels) than they would be for a ductile material of corresponding strength because of generally large shear angles and lower resistance along the tool face.

Ductile materials produce continuous, curled chips. With low friction and high cutting velocities, particularly with material of low work-hardening capacity, a thinner, more tightly curled, less distorted chip is produced. Chip-breakers are required in order to break up the chip. High frictional resistance to flow, low shear angles, and materials of high work-hardening capacity are associated with large distortions during cutting, and are not as big a problem to break up the chip. Additions of lead, sulfur, and phosphorus to low-carbon steels help to break up chips, reduce built-up edge, and improve surface quality.

Today’s cutting fluids are tailor-made tools that require careful selection. They contain a wide choice of special chemical agents designed to supply a definite degree of lubricity, surface activity, stability activity, stability, and anti-weld properties.

An effective coolant should:

cool the work-piece and the tool,

reduce friction,

provide anti-weld properties,

wash away chips,

protect the work from rusting, and

reduce wear.

All metalworking fluids eventually become contaminated and require disposal, even if the user has selected a high-quality fluid and has practiced proper fluid management. With this in mind, some simple coolant-system cleanout procedures will also be recommended which can extend coolant and tool life, as well as improve the entire broaching operation.

Heat Removal

SECTION 3.1
Heat Removal

The relative importance of each function of the coolant depends upon (a) the material being machined, (b) the cutting rate, (c) the tool geometry, and (d) the finish required. No matter how you look at it, however, the primary function of a cutting fluid is to control heat near the cutting edge.

An inverse exponential relationship between the temperature at the cutting edge and the life of the tool was first defined by Taylor, and subsequently expressed as:

where

T = tool life in minutes,

t = temperature at the tool/chip interface (degrees C),

n = an exponent dependent upon the tool material (approx. 20-25), and

k = a constant dependent upon the workpiece and tool material.

This equation has been substantiated by experimentation. Small changes in the tool/chip interface temperature will produce large changes in tool life. A cutting fluid with a high specific heat and a high thermal-conductivity, that provides good surface contact, can increase tool life simply by reducing the tool temperature a few degrees. A coolant does this in two ways: (1) by straight forward heat transfer, and (2) by reducing friction.

The Cooler the Better

SECTION 3.2The Cooler the Better

At 33 surface feet per minute (SFPM) and a 0.003″ rise per tooth (feed rate), cutting edge temperatures are measured at 400°C [750°F]. At 100 SFPM, temperatures rise to 600°C [1112°F]. These temperatures are not conducive to good tool life. Poor tool life will occur if there are poor techniques for reducing or preventing high temperatures at the cutting edge.

Now, consider the empirical equation for tool life. It automatically follows that if we had a coolant which removed heat better, reduced friction more, and more effectively cooled the tool, we would improve tool life. It is that simple, but let’s examine some physical problems.

In an attempt to support this theory, refrigerated coolants have been used. Messrs. Boston, Gilbert, and McGee observed tool wear with applied coolant temperatures of 40°F to 150°F for both a water soluble and an oil. The best results, strictly in terms of tool life, were at 40°F with oil. But, at temperatures of 150°F, the soluble gave better tool life. Another scientist, named Palitzch, was not happy with these results as he felt the varying viscosity of the oil confused the results. He worked with compressed air at various temperatures and had the same results. The conclusion? The cooler the better!

The Advantages (And Drawbacks) of Water

SECTION 3.3The Advantages (And Drawbacks) of Water

As was mentioned earlier, the cutting fluid must provide both a cooling and lubricating action. For a liquid to be most effective in dissipating heat, it must have a high specific heat and a high thermal conductivity. Oil is inferior to water in these respects. Water has a specific heat of 1 compared to 0.45 for a hydrocarbon oil. (Lower specific heat means that, when comparing equal weights of oil to water, for a given amount of heat input, water will have a resultant lower temperature.) This is significant because of the small volume involved. But what is even more important, water will transfer heat two to three times faster than oil.

Water sounds so much superior, but there are some real drawbacks. Water promotes rust, has no lubricity at all, and is not very wet. Therefore, water must have things added to make it slippery and wetter. In other words, surface tension is reduced by adding wetting agents, or surfactants. This enables the heat-transferring water to come in closer contact with the metal. Understanding that the heat transferring requirement for a coolant is quite obvious, as it is the most important function of the coolant.

Understanding Lubrication and Adsorbed Film

SECTION 3.4Understanding Lubrication and Adsorbed Film

The lubrication requirements of a coolant are also important, but are much more difficult to understand and accomplish. In lubrication practices, it often happens that sliding surfaces cannot be separated by a fluid film of lubricant because of a combination of high pressure between the parts, rough surfaces, and low viscosity. This is the case with most metal-cutting operations. Under such conditions, metal to metal contact can be reduced by using lubricants containing additives to encourage the formation of an adsorbed film.

This adsorbed film is composed of molecules of the lubricant which have become chemically combined with the surface atomic structure of the metal. This type of lubrication is referred to as boundary lubrication, since the lubrication is located at the boundary, or interface, of the two contacting surfaces. Such films are often three or four molecules thick–a few millionths of an inch. It is for this reason that intermittent point to point metal contact can still be expected. The best protection under this condition is an oil rich in oiliness and high in film strength.

Oiliness is an important concept, and can be thought of as internal lubrication of the oil. Film strength is a natural component of oils, but can be enhanced with additives such as organic compounds containing sulfur, chlorine, or phosphorus.

Penetration of the coolant to the cutting edge

SECTION 3.5Penetration of the coolant to the cutting edge

We should now consider penetration of the cutting fluid to the cutting edge of the tool. It is clear that the coolant has only two main directions in which it can go: (1) between the tool face and the chip, and (2) between the workpiece (part) and the tool land. Studies have shown that pressurized or forced coolant into either one or both of these directions is superior to normal flooding practices.

Even with pressurization, not much coolant flow will run down either of these two paths to the cutting edge, since both the chips and the work piece are moving in directions opposite to the fluid flow that is necessary. Furthermore, the extreme thinness of the labyrinth formed by the surface irregularities discourages copious fluid flow. But, we can do something that is extremely beneficial. (Note that the actual space the coolant must work in is about 0.000050″ in diameter or smaller.)

The chart (Figure 3-1) below shows a comparison of the droplet sizes of various types of metalworking fluids.

Decreasing Particle Size

Normal Emulsion

Semi-Synthetic Emulsion

Synthetic Emulsion

Particle sizegreater than 0.000,004″

Particle size between
0.000,004″ and 0.000,000,400″

Particle sizeless than 0.000,000,040″

Typical of most cutting fluids, usually ranging from 0.0002″ to 0.000,080″

Described as colloidal solution particles

A true chemical solution, with molecular and ionic dispersions

Figure 3-1.
Coolant droplet size comparisons.

Picture a large and a small sphere, or globule, of oil surrounded by water, but separated by a thin layer of emulsifier. The oil phase of the emulsion carries the additives for film strength, oiliness, lubricity, anti-weld, etc. The water phase carries or transfers the heat. Now picture the action at the cutting edge at a single moment in time. The water is the cooling phase and the oil is the lubrication phase.

With smaller oil globule diameters, the cooling and lubricating will take place more simultaneously and evenly across the cutting edge than with the larger diameter globules. In addition, a proportionately greater amount of the coolant’s lubrication can physically be moved closer to the place where it is needed. A fluid containing large oil spheres will have lower heat transferring capabilities. The size of the sphere depends on the amount and type of tramp oil present in the fluid, as well as the chemical design of the metalworking fluid.

Why the Solution Does Not Always Have to be Synthetic

SECTION 3.6Why the Solution Does Not Always Have to be Synthetic

Based on the evidence presented this far, it would appear that a synthetic solution type would be the best overall. However, that category is often lacking the running lubrication that broaching requires. It is quite likely that semi-synthetic emulsions will penetrate closer to the cutting edge than the normal emulsion types (and some go all the way due to capillary action). It is also probable that synthetic solutions will penetrate even further due to their smaller particle size. But, because of the high temperatures at the chip-tool interface, the liquid vaporizes and may penetrate further as a gas.

This gas provides a pressure separation between the chip and the tool face and carries to the tool edge, or built-up edge, the additives which provide the adsorbed film. These are the contaminants that prevent clean metal-to-metal rubbing. The proper choice of additives will promote formation of adsorbed films on freshly cut metal. These films are more easily sheared than the parent metal, thereby protecting the tool face from pressure welding

The Function of Additives

SECTION 3.7The Function of Additives

If chlorine is present when the steel is freshly cut, an iron chloride film will result. Iron chloride is a relatively weak solid with a fairly high melting point. As such, it provides an excellent lubricant to keep the chips and the tool apart, thereby reducing chip-to-tool friction and increasing the shear angle.

Sulfur is also used and it, correspondingly, forms iron sulfides. Iron sulfide finds a better niche when the speeds, and resulting temperatures, are relatively higher. It seems that too much sulfur activity at low temperatures will have an abrasive effect on tool faces.

The effectiveness of these films is understood to be limited by their melting points. Iron chloride has a melting point of 1100°F and iron sulfide has a melting point around 1850°F. Both chlorine and sulfur react and function essentially the same way; however, chlorine is more reactive than sulfur and begins to combine with the substrate metal at a lower temperature, thus making it better for low speed operations.

Choosing the Perfect Coolant: There Are No Easy Answers

SECTION 3.8Choosing the Perfect Coolant: There Are No Easy Answers

Of course, there are more aspects to coolants than those presented here. There are no real secrets or off-the-shelf recommendations for a perfect coolant. The coolant that is excellent for broaching high-alloy steel may be very poor for broaching stainless, and vice versa. The action of a cutting fluid in reducing friction depends entirely upon the chemical properties involved, and the fluid must be tailored and adjusted with the correct amount and types of additives. Too weak a reaction and the lubricating film may not form. Too strong a reaction, and both the tool and the work piece may be chemically attacked and worn down.

At low cutting speeds, cutting fluids vary greatly in their ability to reduce chip-to-tool face friction. However, at faster cutting speeds, there is less time for fluid penetration, less time to react, and therefore, less friction-reducing ability. Good cooling and wetting abilities are much more important at faster speeds since the time for chemical reactions to form adsorbed films is measured in milliseconds.

Coolant Contamination and Cleanout

SECTION 3.9Coolant Contamination and Cleanout

Contamination of the coolant can come from a variety of sources, such as hydraulic oil, lubricants, floor cleaners, microorganisms, and dissolved metals, all of which inevitably render the fluid ineffective and unusable. This is true even if reclamation or filtration systems exist that periodically circulate the fluid into “as new” condition.

Close control over fluid maintenance will reduce the need for fluid dumping, cleanout, and recharging, and it will improve broach performance and tool life. A proper cleanout procedure should be followed before the coolant is recharged. Properly conducted cleanouts will:

Lower system bacteria and fungus counts, reduce odor problems, and prevent the inoculation of fresh coolant with rancid or contaminated fluid.

Reduce the need for additives, reduce blockage and clogging of pipes and nozzles, and prevent cross-contamination of different coolants, which may not be compatible.

Cleaning the Coolant System

SECTION 3.10Cleaning the Coolant System

Certain basic procedures should be followed to ensure thorough system cleaning. First, pump the dirty fluid out of the system. Inspect all sumps, tanks, and flumes for cracks and dead areas as well as debris and built-up chip dams that may create pockets of stagnant coolant or may trap dirt and swarf. These areas harbor bacteria that will quickly contaminate the fresh fluid. All blind risers or other dead areas in the coolant recirculation and distribution system must be drained to remove stagnant or contaminated fluid.

Next, inspect all machines supplied by a central system to make sure they don’t have a similar problem. Ideally, machines will be purchased that do not have such dead areas. However, shop maintenance or the machine tool manufacturer can modify them so that all chips and coolant can be flushed from the machining area into the system for removal.

Similar procedures should be applied to free-standing machines, even though their fluid systems are less complex. Stagnant pockets of fluid are even more detrimental to individual machines than to machines in centralized systems, because free-standing machines have smaller sumps.SECTION 3.10.1Selecting the right cleaner

A cleaner specifically designed for system cleanouts will greatly improve the initial cleanliness of a freshly charged system and extend its life. If bacteria-contaminated fluid is quickly dumped and recharged without a good cleanout, the bacterial count will return to its original level in about four days. The biocides in the fresh coolant will quickly be consumed in fighting the multiplying growth of leftover bacteria in the system. As a result, fluid odor will recur quickly. This may require more frequent biocide treatment, which may cause operator skin problems. There will, of course, be increased costs for additives, more frequent dumping, and recharging.

A system cleaner formulated for complete removal of residual bacteria should meet certain basic requirements. It must be noncorrosive to all surfaces it contacts during cleanout. It should provide some degree of rust protection for these surfaces after cleaning and before rinsing with coolant. And of course, the cleaner should be compatible with the coolant to be recharged into the system, so that any remnants of the cleaner remaining will not affect the performance of the coolant.

A mild alkaline cleaner is recommended for coolant systems. Select a cleaner that can be applied effectively at temperatures from ambient to 180o F, so it does not require excessive thermal energy to use. Low foaming is desirable to prevent pump cavitation during recirculation of the cleaner.SECTION 3.10.2Maximizing cleanout benefits

The following precautionary steps can be extremely helpful in extending coolant life and minimizing problems.

Install outlet connections and hoses in the coolant-pumping systems of central-system and free-standing machines so that machine operators can flush accumulated chips into the system or sump daily.

On a daily basis, flush areas that catch and hold coolant with more coolant to prevent stagnation and resultant contamination.

Check flumes weekly. Break up chip dams and remove debris that can trap dirt and become a breeding pocket for bacteria.

There is no known tool material that can completely withstand contact and rubbing at high temperatures and pressures over a period of time without some changes from its original contour. It becomes necessary, therefore, to think of the effect of the controlling factors not only upon the cutting process, but upon the performance of the broach cutting tool, which itself affects the cutting process. For the sake of recognition and understanding of the fundamentals of metal cutting, the effects of changes in the controlling factors have been described without regard to their influence upon such criteria as tool wear and tool life.

Tool Failure

SECTION 4.1

Failure of the broach cutting tool has occurred when it is no longer capable of producing parts within the required specifications. The point of failure, together with the amount of wear that determines this failure, is a function of the broaching objective. Surface quality, dimensional stability, cutting forces, and production rates may alone, or in combination, be used as criteria for tool failure.

It may, for instance, take very little wear to affect surface quality, although the tool itself could continue to remove metal with little, if any, loss of efficiency. In contrast, only a few thousandths of an inch of wear on a wide tooth form might cause such a large increase in the pulling force required that it would result in a loss of dimensional stability, or worse yet, a broken broach.

Tool Wear

SECTION 4.2Tool wear

Tool failure is usually associated with some form of breakdown of the cutting edge. Under proper operating conditions, this breakdown takes place gradually over a period of time. In the absence of rigidity, or because of improper tool geometry that gives inadequate support to the cutting edge, the tool may fail by mechanical fractures or chipping under the load of the cutting forces. This is not truly a wear phenomenon for it can be eliminated or at least minimized by proper design and application.

As a direct result of contact with the work material, there are two major regions on the tool where wear can take place: (1) the face, or front of the tooth, and (2) the land, or top of the tooth.SECTION 4.2.1Face wear

The face of the broach tooth is the surface over which the chip passes during its formation. Wear takes the form of a cavity or crater which has its origin not along the cutting edge but at some distance away from it, yet still within the chip contact area. As wear progresses with time, the crater gets wider, longer, and deeper and approaches the outside edges of the tooth.

This form of wear is usually associated with ductile materials which give rise to continuous, tightly curled chips. If crater wear is allowed to proceed too far, the cutting edge becomes weak as it thins out, and breaks down suddenly. Usually, there is some preliminary breakthrough of the crater at the cutting edge. These smaller, prefatory breaks serve as focal points for the development of notches and nicks on the land. In general, crater wear develops faster than land wear on ductile materials and is the limiting factor in the determination of premature tool failure.SECTION 4.2.2Land wear

Although crater wear is the most prominent in the machining of ductile materials, land (or top) wear is always present regardless of work and tool material, or even of cutting conditions. The land is the clearance face of the cutting tool, along which the major cutting edge is located. It is the portion of the broach that is in contact with the work at the chip separation point and resists the machining velocity forces. Because of the clearance, initial contact is made along the cutting edge. Land wear begins at the cutting edge and develops into a wider and wider flat of increasing contact area, called a wear land.

Materials that do not form tightly curled, continuous chips promote little if any crater wear on the tooth face. Land wear then becomes the dominant factor in tool failure. In the case of nearly all broach cutting tools, the wear land is in direct contact with the finished surface, and usually becomes the basis for failure, even on ductile materials, particularly if surface finish specifications are controlling factors in the process. Quite often, land wear is accompanied by a rounding of the cutting edge, particularly in the broaching of abrasive materials. The rounding of the cutting edge is significantly greater on form broaches, especially on sharp corners, where corner wear occurs much more rapidly than wear at the land-face intersection (see Figure 4-1). This results in large increases of cutting forces which, if allowed to accelerate unchecked, could lead to tooth fractures.SECTION 4.2.3The Causes of Tool Wear
All of the evidence indicates that tool wear is a complex phenomenon and is influenced by many factors. The causes of wear do not always behave in the same manner, nor do they always affect wear to the same degree under similar cutting conditions. The causes of wear are not fully understood. In recent years, great strides have been made by various researchers. Even though there is some disagreement regarding the true mechanisms by which wear actually takes place, most studies agree that there are at least five basic causes of wear:

Abrasive action of hard particles contained in the work material.

Plastic deformation of the cutting edge.

Chemical decomposition of the cutting tool contact surfaces.

Diffusion between the work and tool material.

Welding of asperities between the work and the tool (attrition).

The relative effects of these causes are a function of cutting velocity or cutting temperatures. Investigations have also been made on other possible causes such as oxidation and electrochemical reactions in the tool work contact zone.

The most important factor influencing tool wear is cutting temperature. Of the five basic causes of wear, temperature has considerable effect in all but one. Cutting temperatures are important for two basic reasons: (1) most tool materials show rapid loss of strength, hardness, and resistance to abrasion above some critical temperature, and (2) the rate of diffusion between work and tool materials rises very rapidly as temperature increases past the critical point.

Analytical and experimental methods have been used to show that the average peak temperatures at the tool-chip interface occur near the point where the chip leaves the tool surface on the tooth face. Crater wear appears greatest at this point. The rate of wear increases very rapidly beyond a critical temperature. High-speed steel tools begin to lose their properties very rapidly at approximately 1100oF. Chemical decomposition and diffusion will not occur at any appreciable extent until the critical temperatures are reached.

Identifying the Types of Tool Wear

SECTION 4.3Identifying the types of tool wear

There are no known materials or coolants that can completely resist or prevent tool wear. The contact and rubbing at high temperatures and at high pressures will change the original tool contour over a period of time. As a result of direct contact with the work material, we discovered in Section 4.2.3 that there are five major types of wear patterns usually found on a broach.SECTION 4.3.1Abrasive wear

Abrasive wear requires the presence of particles in the workpiece that are harder than the matrix of the tool. This type of wear may be explained by the fact that hard particles (sand, inclusions, carbides, etc.) in the part material literally gouge or dislodge particles from the tool, causing continuous wear under any cutting condition. The particle track, or scratch, alters the geometry of the cutting edge and is not considered a normal cutting condition. At higher cutting speeds, even some of the softer constituents may contribute to the gouging action as a result of higher impact values and reduced tool resistance to abrasion.SECTION 4.3.2Plastic deformation

This wear mechanism is believed to take place at all ranges of cutting temperatures; it arises from the high unit pressures imposed on the tool. This results in a slight depression and bulging of the edge. The net effect is greater tool pressure and increased cutting temperature resulting in further deformation and concluding in edge wipe out. This mode of failure is common when machining hardened materials at high speeds.SECTION 4.3.3Chemical distortion

Chemical distortion occurs through localized chemical reactions at the tool-workpiece interface. These reactions are temperature dependent and result in weakening the bond between minute tool segments and the segments surrounding them. This may occur either through formation of weaker compounds, or in the case of carbide tools, by a dissolving action of the bond between the binder and the individual carbide particles. As a result of this weakening effect, the particles are pulled out from the main body of the tool by the chip or work as it moves past the contact surfaces. Once the critical temperature for this chemical action is reached, the rate of wear is relatively rapid.SECTION 4.3.4Diffusive wear

Diffusive wear is a complex work phenomenon between the work and the tool and results in a rapid breakdown of tool material once critical temperature is reached. There is an alloying effect which weakens the bond for the tool particles and permits them to be pulled out by the chip as it sloughs off. Carbon transfer from the tool material to the workpiece is enhanced at higher temperatures and this greatly contributes to premature tool failure.

Diffusion is dependent on the ability of the tool material to be dissolved into the metal flowing over the cutting surface, rather than on the hardness of the tool material itself. The wear appears to be very smooth and accounts for the formation of craters at speeds below those at which plastic deformation of the tool begins. Rates of diffusion increase rapidly with temperature, the rate typically doubling for every 20oC [78oF] increase. Diffusive wear is the most important wear process responsible for top (land) wear and is reduced by proper coolant choice and application.SECTION 4.3.5Attrition wear

Attrition wear occurs at low temperatures and low speeds where a large built-up edge may be formed. Larger fragments of microscopic size may be torn intermittently from the tool surface, leaving a very uneven worn surface. A built-up edge forms because of a high resistance to chip flow along the tool face, which causes a portion of the chip to shear off as it moves past the tool. With continuous cutting operations using high-speed tools, attrition is usually a slow form of wear, but more rapid destruction of the tool edge occurs in operations involving interruptions of cut, or where vibration is severe due to a lack of rigidity in the machine tool. Adhering metal often completely conceals the attrition-worn surface and, under these conditions, visual measurements of wear may be misleading.

The Effects of Controlling Factors on Tool Wear

SECTION 4.4The effects of controlling factors on tool wear

When we consider the controlling factors upon tool wear, we are concerned either with modifications that influence the cutting process directly for given tool and workpiece materials, or with inherent properties of materials that resist or promote wear. For a given tool and workpiece material combination, cutting temperatures are influenced most by cutting speed, and to a lesser extent, feed and depth of cut. Adjustments in speeds or feeds, or both, will affect tool wear. It may be possible to substitute another tool material or a coated tool material that has inherently better temperature-resistant properties to maintain original or even higher production rates with less sensitivity to temperature failure. The cost of the second material may be higher than that of the first, but it may be more than justified by higher production rates at increased operating temperatures.

Changes in tool geometry that result in higher shear angles, less chip distortion, lower frictional resistance, and thinner chips will lower cutting forces and decrease cutting temperatures, and thus contribute to a reduction in the rate of tool wear for given cutting conditions. Within practical design limitations, rake, relief, radii, etc. should be matched to the application providing the most free-cutting strong geometry that directs the cutting forces in the most rigid section of the workpiece. Heat transfer characteristics may also be adversely affected if the point (cutting edge) of the tool is too thin as a result of high back-off and rake (hook) angles. The heat at the cutting edge does not dissipate as rapidly, and higher temperatures prevail.

Workpiece materials that have relatively high hardness, high shear strength, high coefficient of friction, high work-hardening capacities, and contain hard constituents, promote more rapid wear for given cutting conditions. Materials such as titanium, stainless steel, low-carbon steel, etc. which have poor thermal conductivity, do not dissipate heat from the cutting zone as rapidly as others, and temperature failures are much more common.

When one thinks of broach maintenance, the upkeep of the tool usually comes to mind. However, the handling, storage, and preventive maintenance, as well as the proper care in production can reduce some of the time and cost associated with reconditioning broach cutting tools.

Reconditioning

SECTION 5.1

Each individual broaching application needs to be viewed independently in regard to how much and how often reconditioning may be required. The broached part is the primary factor used to determine the frequency of repair. If the part has a high carbon content, the abrasiveness of the carbon will wear the cutting tool more rapidly. Conversely, the lack of carbon content in the part may provide a softer part which is not conducive to good cutting action, but tends to be pushed by the broach tool (plastic deformation), causing build-up (or galling) on both the cutting edge and flank of the broach tooth.

Convenience is a factor when considering how often to recondition a tool. It is much more practical to recondition or sharpen a round, spline, or plain surface broach. These tools can easily be removed from the broach machine. However, the reconditioning or sharpening of an internal pot broach assembly requires much more time in disassembling and reassembling the pot, which may not be as cost-effective. Therefore, pot broach assemblies are often kept on the machine for a longer period of time.

Optimizing Tool Life

SECTION 5.2

To optimize the tool life of a spline broach, the operator should take the broach off the machine after approximately 2,000 inches has been cut by the broach. If the length of cut in the part is one inch, the broach should be pulled after 2,000 parts. Then, upon inspection and sharpening, the determination can be made whether or not to increase the inches of cut.

If tooth wear on the broach is not excessive (0.003″ to 0.005″) it may be appropriate to increase the number of inches pulled to 3,000. Then another evaluation can be done. This method should be increased until the number of pieces per sharpening is maximized while not causing excessive damage to the broach. If the material being cut is extremely tough and abrasive, a standard of 2,000 inches of total cut may be unattainable. It may then be necessary to sharpen the broaches much more frequently.

The objective when reconditioning a broach tool is to rework the worn or damaged tool so that it performs as closely as possible like a new tool. This process encompasses several operations, because sharpening alone is not the cure for all cases of bad parts. Sometimes a broach may need degalling, restepping of the cutting teeth, deburring, or vapor blasting.

At times, other types of rework, such as welding, may be necessary. When welding is required, the risk of losing the broach is significantly increased. However, if the broach tooth lands have enough life remaining, then the risk may be worth it. This will have to be a judgment call based on the cost of repairing the damaged tool (considering the percentage of tool life left) compared to the cost of purchasing a new tool.

Cutting Tool Nomenclature

SECTION 5.3

The cutting sections of a broach can be broken down into three basic parts. (1) The front section of teeth, or roughing teeth, remove most of the stock. (2) The next section is referred to as the semi-finishing teeth, which gradually reduce the amount of stock taken out. (3) The rear portion of the broach is comprised of the finishing teeth, which create the finish dimensions on the part.

A typical tooth form is shown in Figure 1 below. The pitch is the distance from one cutting tooth to the next. The land is the length of a particular cutting tooth. (Tool life is expressed as a function of the land length.) The depth determines the amount of area available for the part chip, and the face (hook) angle gives a smooth surface for the chip to curl. The face angle radius should be kept constant throughout the life of the broach tool to allow the gullet to perform as required. The back radius of the tooth must blend smoothly with the face angle radius since a mismatch could cause the entering part chip to catch and stick, thereby leading to damage of the broach tool.

The face angle is a variable that is designed to the hardness and type of material being broached. When the broach tool is sharpened, it is critical that this angle be held. The back-off angle provides clearance for the land of the tooth form and is a variable determined by material and part configuration. The back-off angle also needs to be held constant throughout the life of the broach tool.

Figure 5-1.
A Typical Broach Tooth Form.

When is a Broach Tool Ready for Sharpening?

SECTION 5.4

As discussed earlier, there is a general practice which can be altered to suit each particular situation in determining when a broach tool should be pulled for sharpening. The basic spline tooth tends to have a greater corner breakdown than the wear directly along the cutting edge (see Figure 5-2). When a tool has more than the 0.003″ to 0.005″ corner breakdown, an O. D. or top grind is recommended.

This effectively removes the worn areas. If the broach tool is not sharpened properly and worn areas are not removed, the corners will break down further and wear back prematurely. Since part tolerances are usually limiting, the practice of O.D. or top sharpening must be done cautiously and sparingly

Proper Sharpening Procedure (Flat Types of Broaches)

SECTION 5.5

When face sharpening broaches of any type, a 60-80k vitrified bond grinding wheel is commonly used. After sharpening, a polishing wheel (100-210k) is used to blend the back radius with the face angle radius and supply the chip being cut with a smooth surface on which to slide. A recommended grinding speed in face sharpening is 5,000-6,000 surface feet per minute (SFPM). This allows a good cutting action between the grinding wheel and the broach tool.

When sharpening a surface or flat broach, the wheel is traversed laterally across the cutting edge of the tool. The angle of the grinding head should be tilted the same as the face angle on the tool and remove about 0.005″ to 0.010″ stock from the tool face.

Slab broaches can more readily be top ground to eliminate worn corners and then face sharpened and polished. With slab broaches, shims may be used to bring the broach tool back to the proper set-up dimension.

Proper Sharpening Procedure (Round Types of Broaches)

SECTION 5.6

Sharpening a round or spline broach requires following a slightly different procedure. The diameter of the grinding wheel is normally the same as the root diameter (gullet diameter) of the broach tool. This is due to the cutting action between the grinding wheel and broach tool. Since the grinding wheel is now in contact with another cylindrical shaped object and is following an existing face angle, the process becomes slightly more complicated. Now the grinding wheel sharpens with the entering and exiting sides of the wheel, which should cause a cross-hatching pattern on the face of the cutting tooth. This pattern can help determine if the center of the grinding wheel is central to the broach tool; the pattern should be symmetrical.

Figure 5-3.
Formula For Setting Sharpening Head

The formula for calculating the amount of tilt to place on the grinding head is (see Figure 5-3):

Face Angle x (Wheel Diameter / Root Diameter) = Head Angle

If the wheel diameter is the same as the root diameter, the grinding head angle would be the same as the broach face angle. This is not always practical, however, and the formula should then be used. For example, if the required face angle is 15 degrees, the root diameter is 0.850″, and the closest wheel diameter in stock is 2″, the head angle would then be set at 35 degrees (see example).

Example : [15° x (2.00″ / 0.850″) = 35°]

Since an internal broach is a tapered tool, the root diameter at the finishing end of the broach tool is larger than that of the first cutting tooth. The standard practice is to start with a particular set-up (head angle and diameter of grinding wheel). The first tooth to be sharpened is normally the last tooth of the broach. As the gullet decreases with every tooth you progress forward, the size of the grinding wheel also decreases from wear and constant wheel dressing. The number of spaces per interval to move forward and the amount of the wheel to be trimmed comes from experience.

Once the sharpener reaches the front end of the broach tool, a new wheel must be mounted on the head and the process repeated, this time with a polishing wheel. The polishing operation should be given extreme attention to insure the gullet radii are perfectly blended. If the face angle was ground too deeply during the face sharpening operation, the polishing operation will not properly clean up the gullet. The polishing wheel should also barely touch the face, removing a minimal amount of stock.

Throughout the polishing and sharpening operations, the broach should rotate in the same direction as the wheel. This allows greater cutting action, as opposed to a rolling action. The speed of rotation depends on the root diameter of the broach…the smaller the diameter, the faster the speed

Some General Precautions (All Types of Broaches)

SECTION 5.7

An undercut of the face angle radius can cause the chips to pack in the gullet. Usually, when one chip sticks the subsequent chips will also stick, and the gullet will soon become solidly packed. Since there is then no room for more chips, damage will occur. The damage usually appears in the form of broken teeth, with the damage being one tooth, a whole row of teeth, or even a broken broach.

If tooth damage is not too severe, sometimes it is possible to weld a broken tooth and reconstruct it. This is a high risk process since the extreme heat produced by the welding process can soften or crack the broach tool. A much safer method is the elimination of the entire broken tooth and then re-stepping and tapering the teeth following the damaged section. If an entire section of teeth has been broken (on a slab-type broach), a new section can be either bolted or silver soldered into place, and new teeth can be added. As stated earlier, the repair cost must be weighed against the life left in the damaged tool and the cost of a replacement tool.

Removing Sharpening Burrs

SECTION 5.8

In most cases, it is important to remove sharpening burrs. This can most easily be accomplished with a wire brush wheel. The wire wheel should be about 3″ in diameter with 0.008″ diameter wires. The wheel should then be loaded onto a hand-held pneumatic tool and operated at around 17,000 RPM. The operator should begin deburring from the back end of the broach and then proceed toward the front. The rotation of the wire wheel should be such that the bristles pass over the teeth from the back side. This prevents the dulling or damaging of the cutting edge. After deburring, the broach should be washed in mineral spirits to remove all loose burrs and grinding grit.

With the arrival of new high-speed metallurgies (especially particle metal), grinding burrs are increasingly more difficult to remove with the conventional brushing method. The malleability of the steel causes the burr to be pushed over the cutting edge by the wire brush, only to be pushed back after a part has been broached. If the burrs cause streaks or tearing in the part, the broach may have to be sent back to the sharpener for a repeat of the polishing operation, this time using a fine mist to quench the burr as it is formed. The burr will then become brittle, thus making it easier to be removed by the wire brush. A light application with a fine grit Arkansas stone may also help remove the burrs.

Another method of removing sharpening or grinding burrs is vapor blasting. With this process a very fine granular mixture of abrasives is blasted on the back side of the broach teeth. Vapor blasting is the most effective way of removing burrs. However, blasting also removes the acute sharpness of the cutting teeth. The broach should be placed between centers and rotated at 10 to 15 RPM to allow a uniform blasting. The broach must be cleaned with a light oil or mineral spirits to remove the blast grit and powder.

Galling (or Loading)

SECTION 5.9

One of the most common occurrences which causes a need for reconditioning broach cutting tools is galling, or metal pick-up on the broach tool. Gall appears as a dull spotting on the sides of broach teeth when viewed under a bright light. Galling is usually caused by an inferior grade of part material, or contaminated coolant. If left unchecked, galling can compound until part quality suffers, or broach splines and/or teeth become welded together.

Galling is usually removed by hand and, although a tedious job, it is very critical process. First, a coarse stone is used to remove the heavy gall. One side at a time, front to back, the entire broach must be gone over and stoned. (When degalling, care must be taken with the hard stone so the cutting corners of the tooth are not rounded.) Second, a rubberized emery is used to clean and polish the sides of the teeth. The rubber takes the shape of a spline tooth form so that both sides can be completed at the same time. All the existing gall must be removed or else the broach tool will immediately begin to pick up new gall!

Handling and Storage

SECTION 5.10

Careful handling and storage of broach tools is important, especially from a safety point of view. The cutting teeth are very sharp, and some broach bars reach weights of over 500 pounds. When handling broaches, the operator should wear heavy gloves. All hooks and chains should also be checked to avoid accidents.

Precautions against tool damage should be followed. When surface broaches are placed on a table or work bench, the cutting side should be facing upward. Broach tools should be transported on rubber or wooden carts. No metal should ever contact the broach tools. Shipping the broach tools in permanent crates (metal or wooden) specifically made for each particular tool is recommended. Multiple broaches should be separated by wooden dividers. Broach bars should be set on “U” or “V” shaped blocks inside the box to prevent them from rolling. Before shipping a broach (or letting it sit for an extended period of time), it should be coated with a light oil to prevent rust or corrosion.

When handling broach bars with lifts or cranes (such as loading them into a machine), they should be lifted by a rubber coated two-hook set-up. The hooks should be large enough so that the broach bar rests within the hook. The hooks should be placed far enough apart to ensure stability when raising or lowering the broach tool. The chain fall of the crane should be at a height where the chain and broach can never come into contact.

As in the handling of broach tools, there are proper and correct storage methods. Broach rings and dies should be stored in boxes separated by wooden dividers. Slab inserts and keyways should be stored horizontally on wooden shelves. Small broach bars may be stored vertically in wooden racks. Larger broaches may be stored horizontally on racks with the broach contacting wood only. To store long, thin broach tools in a horizontal position is not recommended because this practice promotes bending and warping which can cause problems in use.

Troubleshooting Common Broaching Problems

SECTION 5.11

The following checklist has broken down the most common broaching problems into eight groups. Once the problem is determined, causes can be systematically eliminated and the faulty condition corrected. The checklist applies to all types of broaches including round, spline, involute, pot, serration, square, hexagon, cage, and special form types, as well as slab-type, surface, and keyway broaches.

Part Galling (or Loading)

Broach teeth are mutilated

Repair teeth; heavy face grinding may be necessary.

Check your handling and set-up practice.

Consider follower supports.

Hard fixtures or liner bushings

Use liner bushings having a maximum hardness of Rc 32.

Use softer hole fixtures.

Improper face (hook) angle

Check your sharpening practice.

Verify actual face angle against the broach print.

Review the broach design.

Negative face (hook) angle

Make sure the gullet radius on the grinding wheel is not too large.

Dish-out the back of the grinding wheel.

Deep (or shallow) face angle radius

Check your sharpening practice.

Check the size on the grinding wheel against the size shown on the broach print.

Rounded cutting edges

Check your method of dressing the grinding wheel.

Used the root (gullet) diameter for a steady rest instead of the broach OD.

Burned cutting edge

Check for loading on the grinding wheel.

Do not sharpen off as much stock in one pass with the grinding wheel

Insufficient backoff

Check with the broach manufacturer or supplier.

Verify that the broach print specification is correct.

Incorrectly ground chip breakers

Set the centerline of the chip breaker grinding wheel as far back as possible behind the broach cutting edge.

Poor coolant practice

Use recommended coolant.

Check coolant chemistry.

Make sure coolant is clean (no tramp oil or excess swarf).

Check for leaks of hydraulic oil into the coolant.

Porous forgings

Check the forging practices.

Eliminate variations in the work material by checking all pre-broach operations.

Inclusions in the part material

Use material with a high standard of cleanliness.

Examine material for slag or other foreign matter.

Check the part for pieces of high-speed or carbide tools that may have broken off in a pre-broach operation.

Structure of material

Check the micro structure of the part.

Avoid spheroidal structure (lamellar pearlitic structures are the most desirable).

Streaks in the Part

Metal pickup on lands

Lands may be too thick for the degree of part softness.

Review the broach design.

Face grinding burrs

Remove burrs with a wire brush, fine grit stone, or vapor blast.

Broach a cast iron blank.

Abrasive or vapor blast material on parts

Pickle the forgings.

Alter the part processing sequence.

Clean the broach before using it.

Scale in the coolant

Pickle the forgings.

Install filters or magnetic chip separators.

Excessive scale on the parts

Pickle the forgings.

Specify roto-kut or hole-hog broaches to be designed for this condition.

Abrasion of Broach Surfaces

Excessive scale on the parts

Pickle the forgings.

Specify roto-kut or hole-hog broaches to be designed for this condition.

Scale in the coolant

Remove slag by installing a filtering device.

Cold-worked surfaces

Check microhardness of parts; change processing methods if necessary.

Specify broaches designed for this condition.

Broach condition

Check broach hardness.

Check face angle sharpening.

Broach Breakage

Broach is overloaded

Broach parts with a length of cut no greater than that for which the broach was designed.

Make sure the part is removed before the broach begins its return stroke.

Insufficient backoff angles

Compare backoff angle with the broach print specifications.

Sharp corners on broach pull end

Make certain corners on broach pull ends have chamfers or fillets.

Broach Drifting

Eccentric face grind

Check broach sharpener center runout.

Clean centers of broach.

Small broach body

Specify broaches with body diameters that assure adequate stiffness.

Do not back taper body between splines.

Inadequate broach or part support

Provide adequate supporting means for long or heavy broaches on horizontal machines.

If cast parts are used on a vertical machine, use an equalizing type (dish) fixture.

Use rigid, hard fixturing to prevent part movement during broaching.

Asymmetrical cuts

Correct part processing methods to provide needed concentricities.

Variable part hardness

Correct the hardening processes.

Too few broach teeth in contact

Review the broach design for a minimum of 2-3 teeth in contact.

Non-uniform dullness of broach

Sharpen broach only after cleaning centers and verifying center runout.

Make sure broach is sharpened concentric to outside broach diameters.

Rings In Broached Hole

Surging broach feed

Specify round broaches with staggered tooth spacing.

Specify flat broaches with increased shear angle on cutting teeth.

Excessively sharp broach

Vapor blast the broach to slightly dull the edges.

Lightly stone the outside diameters.

Run a wooden block along the broach teeth.

High backoff angle

Review broach design

Rough part locating face

Provide square locating face on the part, or use equalizing-type (dish) broach fixture

Unsupported broach or part

On a horizontal machine, the broach must be supported until the last cutting tooth is through the workpiece.

Chatter Marks in Broached Part

Too few teeth in simultaneous contact

Review broach design.

Specify shear teeth.

Review face (hook) angles.

Excessive part hardness

Correct hardening practices

Poor fixture design

Design rigid fixtures so that parts are held in firm position during broaching.

Ram surging

Reduce or increase broaching speed.

Check the hydraulic system.

Broach Stuck In Workpiece

Insufficient machine capacity

Provide broaching machine with more tonnage.

Resharpen broach to eliminate dullness.

Redesign tool(s) to require less tonnage.

Clogged chip gullets

Check sharpening and chip breaker practices.

Review broach design.

Power failure

Check power supply.

NOTE: If the broach is stuck in the workpiece, do not try to back the broach out of the workpiece under power. Remove the workpiece and broach and then try to separate them. If that can not be done, the workpiece may have to be broken to separate it from the broach.

Purpose and Function of Toolholders and Workholders

SECTION 6.1

A workholder must position or locate a workpiece in a definite relation to the cutting tool and withstand cutting forces while maintaining precise location.

General Considerations

SECTION 6.2

The design or selection of a workholder is governed by many factors, the first being the physical characteristics of the workpiece. The workholder must be strong enough to support the workpiece without deflection. The workholder material must be carefully selected with the workpiece in mind so that neither will be damaged by abrupt contact.
Cutting forces imposed by the broaching operation vary somewhat in magnitude and direction. However, it usually causes a straight line thrust. The workholder must support the workpiece in opposition to the cutting forces and will generally be designed for each specific application.

The workholder must also establish the location of the workpiece relative to the broach tool. If the operation is to be performed at a precise location on the workpiece, locating between the workpiece and workholder must be equally precise. If the broach tool must engage the workpiece at a specified distance from a workpiece feature such as a line or plane of the workpiece, then the workholder or workholding fixture must establish the line or plane at the specified distance. The degree of precision in the workholder will usually exceed that of the workpiece because of cumulative error.

The strength and stiffness of the workpiece will determine to what extent it must be supported for the broaching operation. If the workpiece design is such that it could be distorted or deflected by machining forces, the workholder must support the affected area. If the workpiece is sufficiently rigid to withstand the machining forces, workholder support at the edge of the workpiece may be adequate. The strength of the workholder is determined by the magnitude of the machining forces and the weight of the workpiece.

Safety requirements must always dictate workholder design or selection. A workholder must not only withstand normal cutting forces and the workpiece weight but may also have to withstand large momentary loads. In machining a cast workpiece, the broach tool might strike an oxide inclusion causing instantaneous multiplication of force. The tool might cut through the inclusion, the tool might break, or the machine might stall. If the workholder broke, however, the tool might impart motion to the workpiece. A workpiece in uncontrolled motion is a missile. The workholder must also be designed to protect workers from their own negligence. Where possible, a shield should be interposed between the worker and the tool.

A workholder should be designed to receive the workpiece in only one position. If a symmetric workpiece can be clamped in more than one position, it is probable that a percentage of workpieces will be incorrectly clamped and machined. Workholders should be designed to prevent incorrect placement and clamping.

It is advisable to use standard workholders and commercially available components whenever possible. Not only can these items be purchased for less than the cost of making them, but they are generally stronger and more accurate.

Chip and burr problems

SECTION 6.3

Chips, burrs, and dirt on locating surfaces cause wear and disturb proper location. Every means must be provided to keep locating surfaces and points free from foreign matter. There are three general methods of chip and dirt control:

Make locators easy to clean. Use small locators. Keep fixtures as open as possible so that supports are readily accessible and visible. Provide easy exit or passage avenues for chip ejection. Pockets or obstructions in a fixture where chips can collect should be avoided.

Make them self-cleaning. Relieve around locating surfaces as an escape for unwanted chips and dirt. Fixed wipers can be used; coolant can flush chips away. Indiscriminate use of compressed air for blowing chips has its drawbacks because chips and dirt can be harmful when blown into ways and other bearing surfaces of machines. Shields and guards control and gather blown chips. Suction is a means of removing light chips and dirt from the work area. Gravity slides can be used to control chips in production setup.

Protect them. Bearing surfaces (such as ways) and indexing mechanisms should be protected. Slides, indexing pins, and buttons should be enclosed.

The grinding wheel, when carefully and correctly used, is a safe cutting tool. But if it is misused or abused, it does have a very dangerous potential. We know that the rim of a wheel running at 6500 surface feet per minute (SFPM) is traveling at a speed of over 70 miles per hour, thus we can understand how dangerous any grinding wheel can be if it is broken by mishandling or misuse.

Rules and recommendations to maintain the safety of grinding wheel use are listed in this document. This document summarizes knowledge gained from literally thousands of man-hours and decades of practical experience with abrasive tools. This document is recommended for employees involved in the mounting or use of grinding wheels, segments, or mounted points. The safety code referred to herein is the American National Standards Institute (ANSI) standard B7.1.

One requirement of the safety code is a prooftest: all sizable grinding wheels must be overspeed tested by the wheel manufacturer before shipment. Typically, this test stresses each wheel to 2-1/4 times the rotational stress of its maximum usage speed. The test destroys any wheel of substandard strength, so it ensures that all surviving wheels are structurally sound. Hence, a precision wheel is perfectly safe to use immediately after its overspeed test. However, it can become very unsafe if it is cracked, chipped, or gouged after that test. So there is good reason to say that grinding wheels don’t break, they are broken.

Maximum Wheel Speed

SECTION 7.1

The maximum operating speed (max RPM) is marked on each wheel, its blotter, or (for small wheels) its container. This number and the outer diameter (D1, in inches) of the new wheel establishes its maximum surface feet per minute (max sfpm) by the following formula:

max sfpm = (p * D1 * max RPM) / 12

This formula allows the max sfpm of the new wheel to set the safe usage limit of the wheel. That is, as the wheel OD is worn down, its RPM may be increased as long as that max sfpm is never exceeded. To maintain the new-wheel sfpm as the OD wears, the following formula may be used:

max RPM 2 = D1 / (D2 * max RPM 1)

Standard and Special Speeds

SECTION 7.2

Mismatching wheel and machine speeds can be dangerous. Recognizing this, wheel and machine manufacturers have agreed over the years on certain standard speeds to which both groups can design. Table 20 of the safety code defines these standard speeds. Although the typical wheel user will have little use of this complex table, he should be aware that it may affect the safety of his operations.

A sure and direct way for the wheel user to avoid mismatching is to routinely specify the actual speed of his machine on all orders for grinding wheels. In the absence of a speed call-out, the supplier will generally supply a standard-speed product. The max RPM marking of the wheel alerts the user not to use the wheel if the machine speed exceeds that standard speed. If such a potential mismatch occurs, return the wheel along with a notation of the actual machine speed.

To avoid speed mismatches, section 7 of the safety code recommends that speed checks be made periodically on all grinding machines. Specific times to do this are cited there by machine type.

A “special” speed is, by definition, any speed that exceeds the appropriate standard speed listed in Table 20 of the safety code. To maintain safety at these higher speeds, section 8 of the code places special requirements on the machine builder, the wheel manufacturer, and the wheel user. The chief responsibility of the user is to make certain that all guarding and other safety devices supplied with the machine are used and maintained in good working order, and that all safety precautions specified by the machine manufacturer are followed.

It is strongly recommended that users do not attempt to increase the wheel speed of any grinder beyond its original design value, nor alter its original guarding. Any intended modification of other safety-related equipment should first be discussed with and/or approved by the manufacturer.

Standard and Special Needs

SECTION 7.3

All grinding wheels can be broken or damaged and should be handled as fragile items. The as-manufactured strength of wheels depends upon their bond type, grit size, grade, shape, and size. However, careless storage, handling, transport, mounting, or usage can degrade this original strength by breaking or weakening the bonding material that holds the wheel together.

Inspection. Wheels should be visually inspected for chips, cracks, or other indications of rough handling. This should be done when they are first removed from their shipping container, or as soon as damage to the container is noticed. Immediately discard or return to the manufacturer any wheel showing such damage. It is important that these wheels be segregated so they cannot possibly be used.

Handling, In-Plant Transport. All wheels are impact-sensitive and should be protected from sudden shocks. They should not be moved or supported by metal devices, e.g. lift truck forks or hoist hooks, unless the metal contact areas are covered by resilient material. Any wheel that is dropped should be considered damaged. Do not roll wheels on the shop floor–cover the path with clean, staple-free cardboard. Do not lay wheels directly on any metal, brick, or similar hard, rigid surface.

Storage. Store wheels on resilient material and only in places that are dry and free from extremes of temperature and humidity. Water-soaking can gradually weaken resinoid wheels; telephone the manufacturer for advice if this happens. Both resinoid and vitrified wheels should be considered unsafe if they are wetted and then subjected to freezing temperatures. A ring test should not be relied upon to detect the widely distributed damage due to freezing a damp wheel

Mounting Grinding Wheels

SECTION 7.4

General. History proves that grinding wheel breakage can result from many causes. However, for precision grinding wheels, one factor stands out. Careful analysis of many precision wheels broken during use shows that improper mounting is involved in more than 50% of all breakage’s. For this reason, each of several methods of mounting precision wheels are individually discussed in the following sections.

In all cases, metal machine components must not press directly on abrasive surfaces. A blotter is to be used between the abrasive and any type of clamping flange or device. Similarly, when a spacer is used beside or between wheels, a blotter must be placed between each spacer and its mating abrasive surface. The spacer’s clamping surface(s) must conform to the same geometric requirements that the safety code places on that machine’s flanges. For example, for multi-screw flanged machines, any spacer used adjacent to a grinding wheel should be relieved near the bore, be the same OD as the flanges, and meet their flatness requirement.

To detect possible damage during in-plant storage, handling, or transport it is important that all wheels be thoroughly inspected immediately before mounting and at the mounting site. This means they should be inspected visually all over, despite any earlier visual inspections, and ring tested. Exceptions are segments, plate-mounted and nut-inserted wheels, mounted points, and some small (less than 4″) wheels; these are difficult or impossible to ring test.

To ring test a wheel that is small enough to hold in one hand, the wheel should be suspended from the hole on a finger. If the wheel is too heavy, it can be supported by a hoist. If it is not possible to suspend the wheel freely in the air, it can be rung while in a vertical position on a clean hard floor. The wheel should be dry and free from sawdust or other packing material.

Tap the side of the wheel gently with a nonmetallic instrument (the handle of a screwdriver or a wooden hammer) about 45 degrees to one side of the vertical centerline and 1 or 2 inches inboard from the OD. Rotate the wheel about 45 degrees in its suspension and tap again. A good vitrified wheel will have a clear metallic ring. A resinoid wheel may not give a clear ring and must be visually inspected for cracks. Large, thick wheels may be ring tested by striking the wheel on the periphery rather than the wheel side.

If there is any reason whatsoever to suspect that a wheel has been damaged, do not mount it. But also do not use the ring test to “disprove” the suspected damage. The ring test should be used to detect unknown cracks but not to discount other evidence of damage–it is not infallible. When damage is suspected, the only safe options are to destroy the wheel beyond further use, or return it to its manufacturer for re-inspection.

Arbor Mounted. New, clean blotters should be used on both sides of the wheel. They assure even pressure on the flanges around the arbor hole. Be careful not to wedge the blotter within the wheel bore.

Flanges and spacers should be of equal diameter (inspect with a straightedge), flat, and free of nicks. Nicks and other protrusions should be carefully removed with a file and stone. The diameter of the flanges should be at least one-third the diameter of the wheel. See the safety code for exceptions, flange dimensions, and spindle diameters.

The wheel should slide easily onto the arbor. If it doesn’t, do not force the wheel. Try to use a larger, wider, or narrower wheel than the size for which the machine was designed.

If the wheel has the word “TOP” on it, rotate the wheel so that “TOP” is at the 12 o’clock position when the clamping nut is tightened. This will keep the hole clearance on the bottom, as it was when the wheel was manufactured. This provides better concentricity and balance, and means less initial truing.

The nut should be uniformly tightened only enough to keep the wheel from slipping; do not use a hammer because over-tightening can warp the flanges, putting high bore stresses in the wheel. The nut should tighten in the opposite direction from wheel rotation!

Collet and Flange. Follow all general instructions (see Mounting Grinding Wheels-General and Mounting Grinding Wheels-Arbor Mounted) down to tightening the nut.

First, tighten the screws with slight finger pressure only. Then tighten to 15 ft. lbs. with a torque wrench. Follow a criss-cross sequence of tightening the screws. Repeat this tightening procedure at least three (3) times or more until all screws are pulled to the prescribed ft. lbs. figure. Wider wheels may require more; the torque recommended by the machine manufacturer should be followed if it is other than 15 ft. lbs.

Excessive tightening may spring the flange and collet. Warpage will reduce the contact area between the flanges and the wheel, and produce dangerously high bore stresses.

Additional precautions are necessary for mounting large wheels on multi-screw flanges. These are covered in the next section.

Large Wheels (Including Centerless). No large, heavy wheel should be mounted manually. The lifting effort required precludes accuracy of alignment and smooth mating of the wheel bore and spindle. Both of these are essential to maintain the safety of the wheel. The initial contact between the spindle and the bore is likely to be jerky and can cause chipping or other damage to the wheel. Some cocking is almost unavoidable. A power lift should be used to avoid these problems.

Some large centerless grinders require removal of the spindle from the machine to change wheels. The machine manufacturer usually provides special fixtures to do this job. Power lifts for these fixtures are recommended. Chainfalls do no usually provide small enough steps of vertical movement to allow accurate alignment, so cocking of the spindle is likely and this endangers the wheel bore. Also, both spindle and wheel bore should be horizontal during mating. This allows the person mounting the wheel to feel any resistance to entry of the spindle into the wheel bore, so he can avoid forcing or cocking. He does not have these factors under his control if mounting is done vertically: the weight above the spindle or wheel is usually enough to chip or crack the wheel, and this is likely unless alignment is absolutely perfect.

Mounted Wheels. The length of overhang of the wheel beyond the chuck has a bearing on the maximum safe speed. The safety code or the Grinding Wheel Institute’s “Mounted Wheels-Safe and Efficient Operation”, latest edition, should be consulted to be certain maximum safe speed is not exceeded.

Guards

SECTION 7.5

Always use a guard. See that it is properly mounted and that all screws are in place and tight.

If the wheel should be broken because of accident or neglect, make sure any damage to the guard is repaired or the guard is replaced. A damaged guard is not much protection. All guards should conform to Section 4 of the safety code.

Starting the Wheel

SECTION 7.6

Stand to one side whenever you start a wheel. Allow new wheels to run at least one minute before grinding.

Safety Glasses

SECTION 7.7

Protect your eyes when grinding. You can lose your glasses and get another pair, but you will lose your eyesight only once.

Grinding Fluid

SECTION 7.8

Turn on the grinding fluid only after the wheel has reached speed, and turn off the fluid before turning off the wheel. Running the fluid on a stopped wheel will create a heavy side and on out-of-balance condition.

Safety Guide

SECTION 7.9

Grinding wheels improperly used are dangerous, but grinding is a safe operation if the few basic rules listed below are followed. These rules are based on material contained in the ANSI B7.1 Safety Requirements for the “Use, Care, and Protection of Abrasive Wheels”.

Mounting. Correct mounting procedures are essential to the efficient and safe operation of the wheel. It is important that personnel performing the function are fully competent.

Do:

Visually inspect all wheels before mounting for possible damage.

Check machine speed against the established maximum safe operating speed marked on the wheel.